Silicon carbide (SiC) has a number of theoretical and practical advantages that make its use desirable in microelectronic devices. These advantages are fairly well known and include a wide band gap, a high breakdown field, high thermal conductivity, high electron drift velocity, excellent thermal stability, and excellent radiation resistance or "hardness." These advantages have been recognized and described thoroughly in the patent and nonpatent literature.
One of the chemical advantages of silicon carbide is its ability to form a stable and well understood oxide, namely silicon dioxide (SiO.sub.2), that can be used to passivate silicon carbide structure and devices. As known to those familiar with electronic devices, an appropriate thermally-grown oxide passivation layer provides an associated advantageous oxide-semiconductor interface that largely eliminates the presence of dangling bonds (sometimes referred to as dangling valences) on the semiconductor surface, and thus largely eliminates the associated problems such as interface charges and traps.
There are, however, some problems that arise from silicon dioxide passivation on silicon carbide because of certain properties of silicon carbide. In particular, a common p-type dopant for silicon carbide is aluminum. Although aluminum gives the highest p-type conductivity in silicon carbide, it recently has been discovered that the presence of aluminum incorporated into the thermally grown oxide passivation layers on p-type silicon carbide tend to cause high fixed oxide charge and high trap density at the silicon dioxide-silicon carbide interface. If the aluminum concentration is sufficiently high (for example when an oxide is grown on p+ silicon carbide) the resulting oxide can have very high leakage currents, rendering it disadvantageous or even useless for passivation or electrical isolation. This problem does not occur when passivating silicon with silicon dioxide because aluminum is not a common dopant for silicon.
As a result of these characteristics, when thermally grown oxides have been used to passivate structures such as mesa p.sup.+ n.sup.- junctions in silicon carbide, the resulting devices tend to demonstrate excessive leakage currents in reverse bias at relatively high voltages (i.e., greater than 50 volts). This leakage current is apparently caused by the poor quality of the passivation on the p.sup.+ side of the junction, causing what effectively amounts to a short circuit around the junction. In some p-channel MOSFETs (metal-oxide-semiconductor field effect transistor), the gate contact has been observed to short entirely through the oxide where it overlaps the p+ source and drain wells.
Additionally, in n-channel MOSFETs, where the electrical integrity of the oxide layers over the aluminum doped p-type channel region are extremely important, the high interface trap density and fixed oxide charge tend to cause the transistors to have high threshold voltages, low transconductances, low channel mobilities at room temperature, and all of which properties tend to change dramatically with temperature. As these MOSFETs are heated, their behavior improves because the increasing density of thermally generated carriers tend to fill the interface traps.
Another problem that arises from the difference between silicon carbide and silicon is that carbon-oxygen compounds are generated by the oxidation of silicon carbide during passivation that are not generated during passivation of silicon. Although not known for certain, these carbon-oxygen species may have their own degrading effect on the electrical integrity of silicon dioxide layers grown on silicon carbide, possibly contributing to fixed oxide charge and premature electric breakdown or wear out.
Finally, earlier work has demonstrated that n-type dopants such as nitrogen tend to pile up severely during thermal oxidation of silicon carbide, resulting in an interfacial concentration more than ten times higher than the bulk of the material. Such dopant pile up could additionally have a profound effect on the electrical characteristics of devices such as MOSFETs.
Therefore, there exists the need to develop a method for passivating silicon carbide device structures advantageously with silicon dioxide while avoiding the aforementioned problems.